Lo que dice el W3C
<em>
The em element represents stress emphasis of its contents.
The level of emphasis that a particular piece of content has is given by its number of ancestor em elements.
The placement of emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which emphasis is used in this way depends on the language.
<i>
The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, a thought, or a ship name in Western texts.
Checando la definicion de ambos tags pues me doy cuenta que los 2 tienen una relacion con el enfasis. La principal diferencia que noto es que <i>es para enfasis de muchos tipos y tambien sirve para marcar texto de "calidad" diferente(otro idioma,terminos tecnicos,etc) en cambio <em> simplemente sirve para dar <em> enfasis algo asi como que llamar la atencion y es que si llama la atencion para eso esta la etiqueta <b>, me podrian decir en que casos debo usar cada una?
Gracias,
